1. Field of the Invention
The present invention relates generally to an alternator mounted on an engine of an automobile such as a truck, and more particularly to the alternator in which bearing units of a rotational shaft have improved vibration proofing while creep caused in bearings of the units is suppressed.
2. Description of Related Art
An alternator attached to an engine of an automobile so as to be driven by the engine has a housing which is composed of a front frame and a rear frame. Each frame is formed in a cup shape, open spaces of the frames are faced each other, and the frames are attached to each other through a stator formed in a cylindrical shape so as to fixedly hold the stator between the frames. A rotor is disposed in the inside space of the stator. A cylindrical front bearing box (hereinafter, called a front box) having a front bearing is provided in the center hole of the front frame, and a cylindrical rear bearing box (hereinafter, called a rear box) having a rear bearing is provided in the center hole of the rear frame. The bearings of the boxes rotatably hold a rotational shaft of the rotor extending along the axial direction of the alternator. The front portion of this shaft is protruded from the front frame, and the shaft is rotated by the engine through a pulley attached to the front end of the shaft.
The outer race of the front bearing is fixed to the front box, while the outer race of the rear bearing is held in the rear box so as to be movable along the axial direction. More specifically, the rear box has a tubular rear bearing holder. This bearing holder is fixed to a side wall of the rear frame located on the rear side of the rear frame, and the bearing holder and the side wall of the rear frame form a bearing receiving portion. Therefore, the rear bearing is held in the bearing receiving portion so as to be slightly movable along the axial direction. Further, a biasing member is provided in the bearing receiving portion so as to push the outer race of the rear bearing toward the front side of the axial direction. Therefore, this biasing member applies a predetermined pressing force directed in the axial direction to the outer race of the rear bearing to prevent the rear bearing from being moved along the axial direction.
It is well known that a washer is used as the biasing member. This washer is located in a space between the rear bearing and an inner side surface (i.e., a side wall surface) of the side wall of the rear frame. This inner side surface also acts as an inner side surface of the side wall of the rear box. The washer is held so as to be in contact with both the outer race of the rear bearing and the inner side surface of the side wall of the rear frame.
To form this washer, a planar spring material is blanked out to obtain a ring-shaped portion, and this ring-shaped portion is bent so as to be formed in a wave shape along the circumferential direction of the ring-shaped portion. For example, a single layer wave washer having no open space along the circumferential direction is obtained. Further, a single layer wave washer having an open space along the circumferential direction is also known. This washer is formed by bending a planar spring material, formed in a long and narrow shape, in a C shape without connecting the ends of the material so as to form only a single layer and reshaping this spring material, formed in the C shape, in a wave shape along the circumferential direction.
Further, Published Japanese Patent First Publication No. 2005-218235 discloses a multi-layered wave washer having an open space along the circumferential direction. This multi-layered wave washer is obtained by repeatedly winding a planar spring material, formed in a long and narrow shape, in circles without connecting the ends of spring material and deforming the wound spring material having many layers in a wave shape. As compared with the single layer wave washer, the multi-layered wave washer integrally formed improves the creep resistance of the bearing and induces the pressing force applied to the outer race of the rear bearing to be easily set within an allowable range.
In each of the single layer wave washer having an open space and the multi-layered wave washer having an open space, the washer body has end portions at different positions in the direction perpendicular to the plane formed by the washer body. When the washer is deformed to be flattened, the end portions are located on the same plane. To prevent the end portions from overlapping with each other, it is required to space the end portions by 2 or 3 mm on the same plane from each other.
However, this space between the end portions easily causes the intertwinement of washers during the thermal treatment and surface finishing performed after the shaping of the washers in mass production. Particularly, as the number of washers intertwining one another is increased, the step for separating the washers from one another is complicated, and it is sometimes required to manually separate the intertwining washers from one another. In this case, the number of steps for producing each washer is extraordinarily increased.
Further, in case of the single layer wave washer having no open space along the circumferential direction, spring characteristics such as a spring constant are rapidly changed with the deformation of the washer, so that the washer located between the rear bearing and the inner side surface of the side wall of the rear frame cannot stably apply a pressing force to the outer race of the rear bearing.